The sense organs — eyes, ears, tongue, skin, and nose — help to protect the body. The human sense organs contain receptors that relay information through sensory neurons to the appropriate places within the nervous system.
Each sense organ contains different receptors.
General receptors are found throughout the body because they are present in skin, visceral organs (visceral meaning in the abdominal cavity), muscles, and joints.
Special receptors include chemoreceptors (chemical receptors) found in the mouth and nose, photoreceptors (light receptors) found in the eyes, and mechanoreceptors found in the ears.
Oooh, that smell: Olfaction
Olfactory cells line the top of your nasal cavity. On one end, olfactory cells have cilia — hair-like attachments — that project into the nasal cavity. On the other end of the cell, are olfactory nerve fibers, which pass through the ethmoid bone and into the olfactory bulb. The olfactory bulb is directly attached to the cerebral cortex of your brain.
As you breathe, anything that is in the air that you take in enters your nasal cavity: hydrogen, oxygen, nitrogen, dust, pollen, chemicals. You don’t “smell” air or dust or pollen, but you can smell chemicals. The olfactory cells are chemoreceptors, which means the olfactory cells have protein receptors that can detect subtle differences in chemicals.
The chemicals bind to the cilia, which generate a nerve impulse that is carried through the olfactory cell, into the olfactory nerve fiber, up to the olfactory bulb and to your brain. Your brain determines what you are smelling. If you are sniffing something that you haven’t experienced before, you need to use another sense, such as taste or sight, to make an imprint in your brain’s memory.
Mmm, mmm, good: Taste
The senses of smell and taste work closely together. If you cannot smell something, you cannot taste it, either. Taste buds on your tongue contain chemoreceptors that work in a similar fashion to the chemoreceptors in the nasal cavity. However, the chemoreceptors in the nose will detect any kind of smell, whereas there are four different types of taste buds, and each detects different types of tastes: sweet, sour, bitter, and salty.
A common misconception is that the little bumps on your tongue are the taste buds. As with all misconceptions, this idea is wrong, too. The little bumps on your tongue are called papillae, and the taste buds actually lie down in the grooves between each papilla.
Foods contain chemicals, and when you put something into your mouth, the taste buds in your tongue can detect what chemicals you are ingesting. Each taste bud has a pore at one end with microvilli sticking out of the pore, and sensory nerve fibers attached to the other end. Chemicals from food bind to the microvilli, generating a nerve impulse that is carried through the sensory nerve fibers and eventually to the brain.
Now hear this: Sound
The ear not only is the organ of hearing, but it also is responsible for maintaining equilibrium — or balance. To maintain equilibrium, the ear must detect movement. To hear, the ear must respond to mechanical stimulation by sound waves.
The outer ear is the external opening to the ear canal. Sound waves are shuttled through the ear canal to the middle ear. The eardrum sets the mechanics in motion:
When a sound wave hits the eardrum, the eardrum moves tiny bones — the malleus, incus, and stapes — which subsequently move.
This movement is picked up by the mechanoreceptors in the inner ear, which exist on hair cells containing cilia between the end of the semicircular canals and the vestibule.
When the cilia move, the cells create an impulse that is sent through the cochlea to the eighth cranial nerve, which carries the impulse to the brain.
The brain then interprets the information as a specific sound.
The fluid within the semicircular canals of the inner ear moves, and that movement is ultimately detected by the cilia. When the fluid doesn’t stop moving, you can develop motion sickness. The cilia transmit impulses to the brain about angular and rotational movement, as well as movement through vertical and horizontal planes, which helps your body to keep its balance.
Seeing is believing: Sight
When you look at an eye, the iris is the colored part. The iris actually is a pigmented muscle that controls the size of the pupil, which dilates to allow more light into the eye or contracts to allow less light into the eye. The iris and pupil are covered by the cornea.
Behind the pupil is an anterior chamber. Behind the anterior chamber is the lens. The ciliary body contains a small muscle that connects to the lens and the iris. The ciliary muscle changes the shape of the lens to adjust for far or near vision. The lens flattens to see farther away, and it becomes rounded for near vision. The process of changing the shape of the lens is called accommodation. People lose the ability of accommodation as they grow older, prompting the need for glasses.
Behind the lens of the eye is the vitreous body, which is filled with a gelatinous material called vitreous humor. This substance gives shape to the eyeball and also transmits light to the very back of the eyeball, where the retina lies. The retina contains photoreceptors, which detect light.
Two types of sensors detect light:
Rods detect motion. The rods work harder in low light.
Cones detect fine detail and color. The cones work best in bright light. There are three types of cones: one that detects blue, one that detects red, and one that detects green. Color blindness occurs when one type of cone is lacking.
When light strikes the rods and cones, nerve impulses are generated. The impulse travels to two types of neurons: first to bipolar cells and then to ganglionic cells. The axons of ganglionic cells form the optic nerve.
The optic nerve carries the impulse directly to the brain. Approximately 150 million rods are in a retina, but only 1 million ganglionic cells and nerve fibers are there, which means that many more rods can be stimulated than there are cells and nerve fibers to carry the impulses. Your eye must combine “messages” before the impulses are sent to the brain.
A touchy-feely subject: Touch
The skin contains general receptors. These receptors can detect touch, pain, pressure, and temperature. Throughout your skin, you have all four of these receptors interspersed. Skin receptors generate an impulse when activated, which is carried to the spinal cord and then to the brain.
The skin is not the only tissue in the body to have receptors, however. Your organs, which are made of tissues, also have receptors. Joints, ligaments, and tendons contain proprioceptors, which detect the position and movement of the limbs.